Plates are already known which are constituted of a thermoplastics resin reinforced with continuous or cut glass fibers, and which are produced by extrusion-laminating or by hot-densification of sheets, such as described in U.S. Pat. Nos. 4,645,565 and 4,451,539 whose teachings are incorporated herein by way of reference. These plates are designed to be transformed by molding-stamping or by thermo-shaping and to this effect they have to undergo a pre-heating treatment.
Among preferred pre-heating methods, infrared or hot air drying are the most used in industry and offer a very oxidizing environment. During that operation, the initially dense plates undergo a phenomenon of expansion as the melting of the resin progresses, this phenomenon being due to the fact that the glass fibers resume their original spatial shape. Such expansion results in a drop in the thermal conductibility, this entailing an overheating of the surface of the materials and their deterioration in the case of intensive supplies of energy. Yet, in order to reach the target rate of transformation, the supply of energy has to be intensive and therefore the material to be transformed has to have a good thermal resistance, clearly above that conventionally used in the other plastic transformation methods, such as injection and extrusion. Therefore, the level of thermal resistance which is required imposes the necessity of introducing a large quantity of thermal stabilizer, during the preparation of the material. It is well known to anyone skilled in the art that such proportions, i.e. several percent by weight, are to the detriment of the mechanical properties, of the smell, of the external appearance, etc. Moreover, such adjunction considerably increases the costs.